所有的表达式对象都具有接口：IExpression
IExpression接口的定义：
Public Interface IExpression
''' -----------------------------------------------------------------------------
''' <summary>
''' 返回条件语句字符串
''' </summary>
''' <returns></returns>
''' <remarks>
''' </remarks>
''' <history>
''' [zqonline] 2006-06-06 Created
''' </history>
''' -----------------------------------------------------------------------------
Function Expression() As String
''' -----------------------------------------------------------------------------
''' <summary>
''' 返回条件语句所对应的Command参数列表
''' </summary>
''' <value></value>
''' <remarks>
''' </remarks>
''' <history>
''' [zqonline] 2006-06-06 Created
''' </history>
''' -----------------------------------------------------------------------------
ReadOnly Property Parameters() As DBAccessLayer.Parameters
''' -----------------------------------------------------------------------------
''' <summary>
''' 设置此条件类所对应的数据源
''' </summary>
''' <value></value>
''' <remarks>
''' </remarks>
''' <history>
''' [zqonline] 2006-06-06 Created
''' </history>
''' -----------------------------------------------------------------------------
Property DataBaseType() As DBAccessLayer.DataBaseTypeEnum
End Interface
表达式类(也就是代码sql语句的一个条件)
当前系统有：
CustomExpression自定条件语句执行比较复杂的条件，如:select 1 from table1 where table1.id=table2.id
FieldExpression字段与字段的条件，如：select * from table1 as a,table2 as b where a.id=b.id
ValueExpression 字段与值的条件，如：select * from table where id=2
FunExpression 实现在函数的条件，如：select * from table where left(name,2)='zq' or select * from table1 as a,table2 as b where left(a.name,2)=left(b.name,2)
命名空间： Expression.Fun定义了常用的sql函数类如：sum,count,avg,left,right,substring,replace,year,month,day它们都需要实现其接口：ISQLFunction
Public Interface ISQLFunction
''' -----------------------------------------------------------------------------
''' <summary>
''' 返回函数对应的语句
''' </summary>
''' <returns></returns>
''' <remarks>
''' </remarks>
''' <history>
''' [zqonline] 2006-06-06 Created
''' </history>
''' -----------------------------------------------------------------------------
Function FunctionDefine() As String
''' -----------------------------------------------------------------------------
''' <summary>
''' 返回或设置数据源类型
''' </summary>
''' <value></value>
''' <remarks>
''' </remarks>
''' <history>
''' [zqonline] 2006-06-06 Created
''' </history>
''' -----------------------------------------------------------------------------
Property DataBaseType() As DBAccessLayer.DataBaseTypeEnum
End Interface
这样对于系统没有实现的函数，以后可以扩展。
ExpressionGroup类是一个特殊的表达式类，也实现了接口IExpression。具有两个方法一个是Add(byval ie as IExpression)，另一个是AddGroup(byval ie as IExpression)
ADD添加一个表达式到ExpressionGroup中,语句示例：where a.name=b.name and a.id>b.id
AddGroup添加一个表达式到ExpressionGroup中，并打上括号，可以实现这样的语句,语句示例：where a.name=b.name and (a.id>b.id or a.age>b.age)
由于ExpressionGroup对象也实现了接口IExpression.所以也可以添加ExpressionGroup
嗯，今天就完成了这部份的设计